1. Field of the Invention
The present disclosure generally relates to an image forming apparatus including a belt extended by a plurality of rotatable supporters and a belt drive control unit for controlling a driving of the belt, and a program for a belt drive control unit.
2. Description of the Background Art
Typically, image forming apparatuses employ either a direct transfer method or an indirect transfer method for forming a color image on a recording medium. In the direct transfer method, toner images formed on a plurality of photoconductors are directly transferred to a transfer sheet. In the indirect transfer method, toner images formed on a plurality of photoconductors are transferred to an intermediate transfer member, and then transferred to a transfer sheet.
Such image forming apparatuses include a plurality of photoconductors, arranged in tandem, for forming latent toner images of yellow (Y), magenta (M), cyan (C), and black (K) and for developing the latent images thereon. Such plurality of photoconductors is disposed so as to face a transfer sheet or an intermediate transfer member. Toner images are transferred from the photoconductors to the transfer sheet, moving in one direction, in the direct transfer method, or to an intermediate transfer member, moving in one direction, in the indirect transfer method.
In such image forming apparatuses, an endless belt is used to support and move the transfer sheet in the direct transfer method, and to receive toner images from the photoconductors in the indirect transfer method. Four photoconductors may be arranged along the endless belt (e.g., transfer belt).
In such image forming apparatuses, color-to-color displacement may occur if a moving (or traveling) velocity of the endless belt cannot be kept constant. Color-to-color displacement may be observed as incorrect superimposing of different color images, which causes image failure. Consequently, a high precision drive control of the endless belt is required to move the endless belt at a constant velocity so that color-to-color displacement caused by fluctuations in moving velocity can be prevented.
Typically, such endless belt is extended by a drive roller and a plurality of driven rollers, and one of the driven rollers provided with an encoder that detects fluctuations in rotation speed of the driven roller. Such information is then used to adjust rotation speed of the drive roller to prevent the color-to-color displacement caused by speed fluctuation in the endless belt. Such adjustment may be referred to as feedback control.
Such feedback control is typically accomplished using phase-locked-loop control (hereinafter, “PLL control”). In PLL control, a difference between a target angular velocity of the drive motor and a detected angular velocity of the encoder is computed as angular velocity error, and then a drive pulse frequency for the drive motor is adjusted by applying a control gain, by which the endless belt can be moved at a target speed.
Specifically, when a transport speed of the endless belt for some reason fluctuates, PLL control is conducted so that the transport speed of the endless belt can be adjusted to a preferred speed which can be detected by the encoder, and the encoder outputs a pulse signal to move the endless belt at a constant speed. In general, fluctuation in the transport speed of the endless belt is due to cyclical variation in a support roller or cyclical variation in a roller contacting an outer face of the endless belt, for example.
However, such control may not be effective for keeping the transport speed of the endless belt constant because fluctuation or variation in a thickness of the endless belt, which may be small in absolute terms, nevertheless may be sufficiently large to cause the transport speed of the endless belt to fluctuate. If a transport speed of transfer sheet or intermediate transfer member fluctuates, image quality may be degraded, and images cannot be produced reliably on the transfer sheet because an image-receiving position on the target transfer sheet or the intermediate transfer member may deviate due to such fluctuation. Further, such fluctuation in transport speed may cause undesirable effects when images are reproduced on multiple transfer sheets. Exactly why these things happen can be explained by examining the structure of the transport mechanism in detail, as is done below.
As shown in FIG. 1, it can be assumed that the transport speed V of the endless belt can be determined with reference to a point located at a center portion in a thickness direction of the endless belt at a position where the endless belt is driven by a drive roller, in which the transport speed V can be defined by equation (1),V=(R+B/2)×ω  (1)in which “R” is the radius of the drive roller, “B” is the thickness of endless belt, and “ω” is angular velocity of the drive roller. If the belt thickness B varies, then a position of an effective thickness line of the endless belt (belt effective thickness line), shown in FIG. 1 as a dotted line, changes. If the position of the belt effective thickness line changes, then an effective radius of the endless belt also changes, by which (R+B/2) in the equation (1) changes. Accordingly, even if the angular velocity of the drive roller “ω” is set to a constant value, the transport speed of the endless belt varies. Accordingly, even if the drive roller is rotated at a constant angular velocity, the transport speed of the endless belt varies if the thickness of the endless belt varies. FIG. 2 illustrates a schematic configuration of the endless belt, in which a belt 1010 is extended by a drive roller 1015 and driven rollers 1014 and 1016.
FIG. 3 illustrates a relation between a thickness fluctuation or variation of the belt 1010 along an entire length of the endless belt 1010 and a transport speed fluctuation or variation of the endless belt 1010 when the drive roller 1015 is rotated at a constant angular velocity. As a thicker part of the belt 1010 winds around the drive roller 1015, an effective radius of the belt (see FIG. 1) is increased, by which the transport speed of the endless belt also increases as understood from the equation (1). As a thinner part of the belt 1010 winds around the drive roller 1015, an effective radius of the belt 1010 (see FIG. 1) is decreased, by which the transport speed of the belt 1010 also decreases as understood from the equation (1).
Further, FIG. 4 illustrates a relation between a thickness fluctuation or variation of the belt 1010 at the driven roller 1014 and a transport speed fluctuation or variation of the belt 1010 detected at the driven roller 1014 when the belt 1010 moves at a constant transport speed. As a thicker part of the belt 1010 winds onto the driven roller 1014, the effective radius of the belt 1010 at the driven roller 1014 is increased, by which the angular velocity of the driven roller 1014 decreases, by which the transport speed of the belt 1010 also decreases as understood from the equation (1). By contrast, as a thinner part of the belt 1010 winds on the driven roller 1014, an effective radius of the belt 1010 at the driven roller 1014 is decreased, by which the angular velocity of the driven roller 1014 increases, by which the transport speed of the belt 1010 also increases as understood from the equation (1).
If the thickness fluctuation in the belt 1010 is confirmed along the entire length of the belt 1010, a transport speed of the belt 1010 detected by an encoder disposed at a shaft of the driven roller 1014 may be a detection error deviated from a target speed.
Therefore, even if the belt 1010 moves at a constant speed, the detection results of the encoder may indicate that a transport speed of the belt 1010 varies from the target speed because variation in angular velocity of the driven roller 1014 is detected due to a thickness fluctuation in the belt 1010 along the entire length of the belt 1010. Accordingly, a conventional feedback control using the driven roller may not be effective in view of a thickness fluctuation in the belt because the speed detection results may falsely indicate a speed fluctuation in the endless belt.
Some related-art approaches for remedying the above-described problem disclose methods for precisely controlling rotation of a belt, in which a drive roller for driving the belt or a driven roller driven with the belt are controlled as below described.
JP-2000-310897-A (reference 1) discloses an image forming apparatus including a transfer belt extended by a drive roller and a driven roller. The transfer belt has a mark to detect the position of the transfer belt movable in one direction. When the drive roller is activated at a constant pulse rate, a transfer belt thickness profile (“thickness fluctuation in the transfer belt”) is obtained along the entire length of the transfer belt. Such thickness fluctuation in the transfer belt may cause a transport speed variation Vh. Then, a “transfer belt speed deviation” (transfer belt speed profile) which can compensate for the transport speed variation Vh is computed and a control signal for the drive motor is generated from a modified pulse rate based on such computation, with the drive motor driven to rotate the transfer belt using the drive roller. Accordingly, a transfer belt speed Vb of the transfer belt can be kept constant.
However, in reference 1, data of speed fluctuation in the transfer belt needs to be collected for each belt rotation. If a control cycle is set short, a large capacity memory may be needed, whereas if the control cycle is set long, feedback control may not be conducted effectively. For example, if the transfer belt has a circumference length of 815 mm, a belt speed of 125 mm/s, and a control cycle of 1 ms, the speed control may be conducted 6520 times per rotation of the transfer belt (815 mm/(125 mm/s×1 ms)=6520 times). Further, if data size of transfer belt thickness per one control is set to 16-bit to improve the control precision, a memory having 100 k bit may be required (6520×16 bit=104,320 bit).
When such speed control is conducted, a memory (e.g., non-volatile memory) may be required for storing data of thickness fluctuation in the transfer belt. Accordingly, even if the data is compressed for storing, and decompressed to a volatile memory when the power is set ON, a larger capacity memory may be required. Accordingly, in addition to a memory used as a working area, another memory may also be required, which increases a total cost of the apparatus.
Further, a thickness fluctuation in the transfer belt may need to be measured as thickness data of the transfer belt, in which a laser displacement gauge may be used. The measured data is input to an image forming apparatus when shipping products or when a service engineer checks an image forming apparatus using an operation panel or the like. However, the thickness fluctuation in the transfer belt needs to be measured at a higher precision of several micrometers (μm) or so, and an input error may occur when inputting data because the amount of measured data may become great.
In view of such drawbacks of reference 1, JP-2006-106642-A (reference 2) discloses an image forming apparatus including a belt drive control unit. A drive motor outputs a drive input signal to a converter unit, in which the drive input signal is converted to angular velocity of a driven roller. A comparison unit compares a drive output signal and the drive input signal (converted by the converter unit) to obtain fluctuation composition caused by thickness fluctuation in one rotation of the belt. Then, a periodic fluctuation sampling unit stores the fluctuation composition caused by thickness fluctuation per rotation of the belt to a memory. An amplitude and phase detector detects amplitude and phase of the belt in a rotation cycle using the fluctuation composition per one rotation of the belt stored in the memory.
Reference 2 discloses an image forming apparatus having a belt unit and a belt drive control unit, which can conduct a belt drive control process during an image forming process, in which amplitude and phase of a belt corresponding to a cyclical thickness fluctuation in the belt can be extracted based on angular velocity or rotation angle displacement having a given frequency.
In reference 2, a detection result at a shaft of the drive roller is subtracted from a detection result at a shaft of the driven roller shaft to obtain the belt fluctuation component having a frequency corresponding to a cyclical thickness fluctuation in the belt. Based on the belt fluctuation component, amplitude and phase of the belt can be is extracted, and a rotation of the drive roller is controlled based on such computed values.
Specifically, a periodical fluctuation component of the belt per one period starting from a virtual home position VHP of the belt can be detected and stored in a memory. The stored fluctuation component can be used to detect amplitude and phase of primary wave and higher harmonic wave. Angular velocity or rotation angle displacement detected by an encoder disposed to the driven roller can be used as belt fluctuation component corresponding to the thickness fluctuation in the belt.
However, a computation process to detect amplitude and phase using zero cross method for fluctuation component, a computation process to detect amplitude and phase of fluctuation component of a previously-determined cycle from a peak value, and a detection process for a component of a previously-determined cycle using quadrature detection all require a given time duration (or a given time-delay). Accordingly, due to such time delay, the detected amplitude and phase may not be applied to a right position of the belt, which needs to be corrected by the detected amplitude and phase. Accordingly, the computed amplitude and phase may not be applied to a right position of the belt, which needs to be corrected based on the computed amplitude and phase. In other words, the computed amplitude and phase may be applied to a position of the belt different from a to-be-corrected position.